This is called the Cascade (how appropriate) from Homestead Design. There is plenty of space on the bottom floor for one car, storage, plus a workbench & counter with sink and burners.
Looks like a decent amount of space upstairs for an apartment/guest room, with a little storage over the bathroom. Adding a bathroom will necessitate having the septic checked out by the county. Ours was installed way back when, and probably needs updating/replacing if we want to get this project permitted. Not cheap, but that's life.
Here's the same plan someone in New York built. They got rid of the additional entry door in front of the bathroom and sided it in board & batten. We can probably save a little on siding since we want paint grade (to match the house) not stain quality. But nothing is ever cheap these days, so we'll just start looking for bids and see how it goes.
P.S. Anyone interested in sponsoring a board, batten or commemorative roof shingle will receive membership in the Seven Trees Fun Plan - A stay in the guest house, homebrew tastings, and a home-cooked meal.